1. Object of the Invention
This invention relates to a buoyant arm for maintaining tension on a riser, in particular a drilling riser, used on a floating or semi-submersible petroleum drilling or production platform.
2. Field of the Invention
The search for new oil aand gas fields has led to increasing amount of drilling activity in offshore locations. Drilling or producing from subsea offshore wells, particularly in deep water, typically requires the use of a floating vessel from which to conduct operations. A "riser" communicates the vessel with the wellhead and must be maintained relatively straight and is, therefore, constantly under tension. A riser operating on a floating vessel in water depths greater than about 200 feet can buckle under the influence of its own weight and the weight of drilling fluid contained within the riser if the riser is not maintained in tension. This axial tension must be applied either to the top of the riser or result from buoyancy attached along the length of the riser. The tension controls the stress in the riser and affects its straightness. It should be apparent that as water depth increases, the axial tension required to provide proper support also increases.
A "drilling riser" is a conduit which serves essentially the same function as does well casing on an onshore well. It encloses the drill string and provides a pathway for the drilling fluids or "mud" to return to the floating drilling platform. The drilling mud is, of course, pumped down through the hollow interior of the drill string, out orifices found in the drill bit, and back up the annular space between the outside of the drill string and the inside of the drilling riser.
There are a number of methods for tensioning risers. Two known ways provide for the use of counterweights or pneumatic spring systems at the top of the risers. Use of a counterweight was an early technique for applying tension to the top of a marine riser. The weight is hung from a wire rope which is in turn run over sheaves and down to the top of the riser pipe. As the vessel moves up and down, the counterweight moves up and down and maintains tension on the top of the riser. The tension on the riser is equal to the weight of the counterweight. The tension varies significantly due to the acceleration of the counterweight when the vessel heaves. Consequently, the method is only practical for calm shallow water locations requiring only low tension.
The pneumatic spring systems replaced the counterweight systems as deep water and severe weather drilling evolved. These devices utilize a cylinder containing compressed air to apply tension to the top of the riser through wire ropes. These devices act as oil damped pneumatic springs. A large air supply maintains a nearly constant pressure above the oil in an air-oil accumulator vessel. The oil then provides pressure via a hydraulic line to the face of a moving piston. As the floating vessel moves up and down, the piston moves in a similar manner and thereby provides a relatively constant force in the lines attached to that piston. A series of sheaves are provided on the tensioner so that the length of the piston stroke will be substantially less than the scope of the vessel movement.
Tensioner systems proposed in the past are subject to several disadvantages; one disadvantage being that the tensioning lines often fail under high tension. Failure in this manner is typically attributed to fatigue caused by the continuous bending of the cable as it rolls back and forth over the sheaves. Another problem is that conventional tensioning systems with the capacity to provide the tension for deep water drilling may be inordinately massive. The mass and the high tension can adversely affect vessel stability.
The inventive drilling riser system with its buoyant arm disclosed herein helps to minimize these problems. The invention may be used with any number of different of drilling vessels, although it is particularly suited with the caisson vessel type which has a vertical chamber extending through the center of the vessel through which a drill string is extended. Other vessels having such a central chamber are known. For instance, the description found in U.S. Pat. No. 3,766,874, to Helm et al, outlines a design for a moored barge to be used in arctic offshore oil drilling. The design has a central vertical chamber through which the drill string is placed. Although no mention is made of a riser or, therefore, any riser tensioner means, certainly such apparatus would be required.
Another drilling structure having a central drilling chamber is described in U.S. Pat. No. 3,771,481, to Goren, and in U.S. Pat. No. Re. 29,478. This vessel is of the type known as "semi-submersible". It is a structure that typically has a long large central column setting upon barge-shaped catamaran hulls. A drilling deck is perched atop the central column from which both drill string and drilling riser are snaked to the ocean bottom. The vessel is known as "semi-submersible" since it has large ballast chambers allowing the vessel to be partly submerged, i.e., so that the hulls are well below the water's surface. Once so submerged, the vessel can be maintained in position by sea anchors. No mention is made of the manner in which the riser is tensioned; nor is any mention made of the method by which the riser is held away from the interior of the central column walls.
Another semi-submersible drilling structure is found in U.S. Pat. No. 4,004,531, to Mott. This vessel contains a central chamber denominated a "caisson" which extends down from the drilling deck to below the water surface and is designed to protect the drilling riser from wave forces. A slip joint is placed in the riser string within the caisson to decouple vertical vessel motion from the riser. A joint allowing pivoting action about the riser is also included in the riser string. The riser tensioner disclosed apparently is of the air pressure type mentioned above. The patent discloses an additional embodiment intended to lessen the load of the mechanical riser tensioner. In the latter embodiment, a number of air-filled rubber bladders are attached to the riser below the water line. The buoyancy of the bladders creates tension on the riser and, depending on the size of the bladders, could either eliminate the need for a mechanical tensioner or at least reduce the size thereof.
A floating or semi-submersible vessel suitable for production or pumping petroleum from a sea bed floor storage facility is disclosed in U.S. Pat. No. 4,212,561, to Wipkins. The apparatus disclosed therein utilizes a central column through which a riser is inserted. The riser is maintained within the center of that column by a set of rollers and floats. The portion of the riser extending from the sea bed floor is made buoyant apparently by collars affixed about the riser itself. The upper end of the riser has a swivel joint at the point it enters the central column to decouple the roll motion of the vessel from the riser.